A molding machine cylinder used for injection-molding or extrusion-molding plastics, etc., is required to be resistant to wear by resins, additives, etc. during thermal molding, and be able to be produced at relatively low cost. To meet such requests, a bimetal-structure molding machine cylinder comprising a lining layer formed on an inner surface of a steel cylinder by a centrifugal casting method, the lining layer comprising tungsten carbide particles dispersed in a nickel alloy matrix, has conventionally been used.
To produce a molding machine cylinder having a lining layer, in which hard particles such as tungsten carbide particles, etc. are dispersed in a nickel alloy matrix, by a centrifugal casting method, a mixed powder of nickel alloy powder and tungsten carbide powder is melted and centrifugally cast in a cylinder, forming a centrifugally cast layer comprising an outer layer (lining layer), in which many hard particles having large specific gravities such as tungsten carbide particles, etc. are dispersed, and an inner layer containing less hard particles (particle-lack layer). Because the particle-lack layer should be removed to expose the lining layer, the particle-lack layer is preferably as thin as possible.
JP 2872571 B discloses a centrifugally cast composite tungsten carbide lining layer comprising 30-45% by weight of tungsten carbide, 35-50% by weight in total of nickel+cobalt, 1% or less by weight of molybdenum, 10% or less by weight of chromium, 1-3% by weight of boron, 1-3% by weight of silicon, 2% or less by weight of manganese, 8-25% by weight of iron, and 1% or less by weight of carbon. This lining layer has excellent wear resistance, because 25-45% by volume of tungsten carbide particles having an average particle size of 6-12 μm are dispersed. However, JP 2872571 B does not describe how to stably control the thickness of a particle-lack layer at all. With the particle-lack layer having uneven thickness, not only the particle-lack layer but also part of the lining layer should be removed for safety margin, resulting in a low production yield. Also, the lining layer containing as much iron as 8-25% by weight has poor corrosion resistance.
JP 4900806 B discloses a molding machine cylinder comprising a lining layer of a wear-resistant, corrosion-resistant alloy, which is formed on an inner surface of a hollow steel cylinder, the lining layer comprising a nickel-containing matrix, and hard particles comprising tungsten carbide dispersed in metal tungsten. JP 4900806 B describes that an area ratio of hard particles in the lining layer is 20-80%, and that the lining layer may further contain 5-20% by area of tungsten boride. JP 4900806 B further describes that tungsten boride is centrifugally separated together with hard particles during centrifugal casting, resulting in proper distances between hard particles in the lining layer. However, it is difficult to stably control the thickness of the particle-lack layer in this cylinder, too. With the particle-lack layer having uneven thickness, the centrifugally cast layer should be cut deep to remove the unevenly thick particle-lack layer, resulting in a low material yield. In addition, special particles containing tungsten carbide dispersed in metal tungsten should be prepared as hard particles, resulting in a high production cost.
JP 5095669 B discloses a lining material centrifugally cast in a cylinder, which comprises 30-45% by mass in total of tungsten boride+tungsten carbide (tungsten boride/tungsten carbide=1 or less, and tungsten boride: 5-20% by mass), 35-50% by mass in total of nickel+cobalt, 1% or less by mass of molybdenum, 10% or less by mass of chromium, 1-3% by mass of boron, 1-3% by mass of silicon, 2% or less by mass of manganese, 5% or less by mass of iron, 1% or less by mass of carbon, and inevitable impurities. JP 5095669 B describes that with an adjusted mass ratio of tungsten carbide powder to tungsten boride powder, the thickness of a particle-lack layer is controlled. However, tungsten boride powder added to the lining material in advance leads to a high material cost, and is dissolved in an alloy melt during casting because tungsten boride is poorer than tungsten carbide in thermal stability. In addition, tungsten boride particles having particle sizes of several micrometers or less, much smaller than the alloy powder in a raw material, are not easily dispersed uniformly. Further, with too large a total amount of tungsten carbide powder and tungsten boride powder, these powders have insufficient fluidity during centrifugal casting, failing to achieve the uniform dispersion of tungsten carbide particles and tungsten boride particles.
JP 2010-99693 A discloses a method for producing a wear-resistant lining layer by (a) adding boride powder including WB or MoB to Co-based or N-based alloy powder containing B and Cr to prepare a mixed powder, (b) charging the mixed powder into a cylinder, (c) melting the mixed powder by heating it to a temperature of 1200° C. or higher while rotating the cylinder at 3 rpm, (d) forming a lining layer by centrifugal casting, in which the cylinder is rotated at a high speed of 2290-2300 rpm, and (e) finishing an inner surface of the lining layer by machining. However, because the mixed powder is melted at a high temperature of 1200° C. or higher in this method, the inner surface of the cylinder is eroded, so that a large amount of molten Fe enters the lining layer. Because the lining layer exhibits low corrosion resistance with a large Fe content, the erosion of the inner surface of the cylinder should be minimized during melting the mixed powder. Though boride powder is added in this method, tungsten boride is partially melted in the alloy melt because of poorer thermal stability than tungsten carbide, resulting in insufficient improvement in wear resistance.